Thermoforming machines to form a thermoplastic sheet by the use of heat and pressure are well known in the art. Conventionally, such a machine is provided with a sheet carriage for conveying a thermoplastic sheet from a heater unit to a pressing unit. In Japanese Patent No. 51-27259 a rectangular thermoplastic sheet to be formed is supported on a sheet carriage in which there is a rectangular frame for clamping the peripheral edge of the sheet. The thermoplastic sheet supported on the carriage is first positioned in the heater unit for heating the sheet to thereby soften it. After the heating step is completed, the sheet supported on the carriage leaves the heater unit and is moved to the pressing unit, wherein the heated and softened sheet is pressed by a pair of molding dies into a desirable fixed shape. In this pressing process, the molding dies are positioned within the carriage. The carriage and its associated sheet need to move in unison during the heating and pressing processes since the carriage clamps the sheet.
A principal disadvantage of the thermoforming machine of the type discussed above is that the carriage cannot be pulled back from the pressing unit while the pressing process is being carried out, since its frame, which clamps the sheet, cannot be moved as the sheet is pressed by the molding dies. Thus, the carriage is an idling condition during a considerable period of the pressing process, and cannot be used for a following sheet during the pressing process for the preceding sheet held on the carriage. This results in a longer time for each operating cycle for thermoforming. It is very important that all processes be accomplished without any idling time.